Prior art research-type EPR spectrometers have generally included a high Q resonant cavity excited by a high quality, relatively noise-free source. A high Q cavity requires a high cavity volume to cavity wall surface ratio, which in turn necessitates a relatively large volume of polarizing magnetic field, thereby requiring a large magnet weighing 1000 kg or more. Heretofore these spectrometers have been designed to obtain a flexible instrument of high sensitivity for a wide variety of samples in a wide variety of applications. This objective has resulted in research-type equipment which is generally considered very expensive.
There are three types of aqueous sample cell geometries that have been used in the past for EPR experiments. In many different cavities at a frequency of 10 GHz a capillary of about 1 mm diameter has proven to be optimum when operating within the constraint that it is desirable to use a cylindrical tube to contain the sample. 1 mm capillaries are furthermore convenient because they are readily available, inexpensive, and precise in size, permitting accurate quantitative measurements. A second type, the so-called flat cell, is described by L. G. Stoodley, J. Electronics and Control, 14, 531 (1963).
Stoodley analyzes a rectangular TE.sub.102 cavity having a nodal plane of zero microwave electric field and maximum microwave magnetic field considering the aqueous sample to be a slab coincident with this plane but of finite thickness extending slightly into regions of finite electric field intensity.
A third type of cell has been described by J. S. Hyde, Rev. Sci. Instru., 43, 629 (1972). Hyde shows that samples with high dielectric loss can be inserted into regions of high microwave electric field intensity providing that the electric field vectors are perpendicular to the surface of the cell. Polarization charges on the surface of the cell tend to prevent the electric field from penetrating the sample.